Abstract

The existence of adaptive immunity in prokaryotes came to light with the discovery of the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) in association with CRISPR-associated (Cas) proteins. This RNA mediated defence system confers resistance against the invading mobile genetic elements such as phages and plasmids. The CRISPR-Cas system operates by forming a ribonucleoprotein complex that comprises of an invader derived small RNA and Cas proteins. Herein the small RNA acts as a guide to recognize the nucleic acid target whereas the Cas proteins facilitate target annihilation. Given the cardinal role adopted by this small RNA, its maturation from the pre-CRISPR transcript forms a pivot for successful adaptive immunity. The mandate to generate the guide CRISPR RNA (crRNA) is fulfilled by specific endonucleases, which process the pre-crRNA transcript in between the repeats to liberate the individual mature crRNA units. Intriguingly, while some endoRNases of the CRISPR system are able to process the pre-crRNA unaided, others require association of additional Cas proteins to form a multi-protein complex, which then process the pre- crRNA. Additionally, some CRISPR variants require auxiliary factors to process the pre- crRNA. The mode of crRNA maturation further diversifies as the endoRNases in CRISPR variants coevolve with repeat clusters that exhibit high diversity in sequence and folding. Therefore, the maturation of specific crRNA requires a distinct mechanistic solution for substrate discrimination by these endoRNases, the understanding of which is essential for appreciating the CRISPR biology. This review highlights the vivid modes adopted by the diverse CRISPR-Cas systems to generate the mature crRNA.